Conventional industrial plants synthesize polycarbonate by mixing together an aqueous solution of a dihydroxy compound (e.g., bisphenol-A) with an organic solvent (e.g., dicloromethane) containing a carbonyl halide (e.g., phosgene). Upon mixing the immiscible organic and aqueous phases, the dihydroxy compound reacts with the carbonyl halide at the phase interface. Typically, a phase transfer catalyst, such as a tertiary amine, is added to the aqueous phase to enhance this reaction. This synthesis method is commonly known as the "interfacial" synthesis method for preparing polycarbonate.
The interfacial method for making polycarbonate has several inherent disadvantages. First, it is a disadvantage to operate a process which requires phosgene as a reactant due to obvious safety concerns. Second, it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the material process is prone to having inconsistent color, higher levels of particulates, and higher chlorine content, which can cause corrosion.
A new method of manufacturing has been developed which avoids several of the problems associated with the interfacial method. Specifically, some newer commercial polycarbonate plants synthesize polycarbonate by a transesterification reaction where a carbonate diester (e.g., diphenylcarbonate) is condensed with a dihydroxy compound (e.g., bisphenol-A). This reaction is performed without a solvent, and is driven to completion by mixing the reactants under reduced pressure and high temperature with simultaneous distillation of the phenol produced by the reaction. This synthesis technique is commonly referred to as the "melt" technique. The melt technique is safer than the interfacial technique because it does not employ phosgene, it does not require a solvent, and it uses less equipment. Moreover, the polycarbonate produced by the melt process does not contain chlorine contamination from the reactants, has lower particulate levels, and has a more consistent color. Therefore, it is usually more desirable to use the melt technique in a commercial manufacturing process.
The melt technique produces polycarbonate which differs from polycarbonate produced by the interfacial method. Specifically, the conventional interfacial method tends to produce polycarbonate which has close to zero branching. Not only is it desirable to have a controlled level of branching for some applications, such as those which require a very high ductility, but also a high level of branching is desireable for other applications which require high melt strength. If any branching is desired in polycarbonate produced by the interfacial process, it must be introduced by adding a branching agent during polymerization because the Fries branching species is not typically present in appreciable amounts in polycarbonate produced by the interfacial method. In contrast, the melt technique tends to produce polycarbonate having a high level of Fries branching species. Accordingly, it would be desirable to produce controlled Fries polycarbonate by the melt technique for certain applications because higher levels of Fries are associated with low ductility. As noted below, Applicants have solved this problem.
Japanese Published Patent Application Number 9-59371 to Teijin (hereinafter the "Teijin Publication") discloses a method for manufacturing polycarbonate by the melt process wherein the polycarbonate contains from 0.001 to 0.3 mole percent of Fries plus a second branching species, but contains at least 0.001 mole percent of the second branching species. Therefore, the Teijin Publication specifies melt polycarbonate having a level of Fries below 0.299 mole percent. However, the Teijin Publication does not teach how to make polycarbonate by the melt process which has a very low level of Fries. In fact, the Teijin publication only discloses a polycarbonate made by the melt process having a level of Fries above about 360 ppm (working example 3) and does not mention catalysts which are effective in significantly reducing Fries content. Moreover, the Teijin Publication does not disclose the advantages of using melt polycarbonate having a very low level of Fries in specific applications.
A general need clearly exists for a polycarbonate made by the melt process which has a controlled level of Fries. A need also exists for a method for making low Fries polycarbonate via the melt process.